Various approaches are described in the literature for fabricating hermetically sealed electrical circuit housings suitable for extended operation in corrosive environments, e.g., in medical devices implanted in a patient's body. For such applications, a housing must be formed of biocompatible and electrochemically stable materials and typically must include a wall containing multiple hermetic electrical feedthroughs. A hermetic electrical feedthrough is comprised of electrically conductive material which extends through and is hermetically sealed in the wall material.
One known approach uses an assembled pin feedthrough consisting of a conductive pin that is bonded chemically at its perimeter through brazing or the use of oxides, and/or welded, and/or mechanically bonded through compression to a ceramic body. Typically, gold is used as a braze material that wets the feedthrough pin and the ceramic body resulting in a hermetic seal. Wetting to the ceramic body requires a deposited layer of metal such as titanium. This layer acts additionally as a diffusion barrier for the gold.
Other alternative feedthrough approaches use a metal tube cofired with a green ceramic sheet. The hermeticity of the metal/ceramic interface is achieved by a compression seal formed by material shrinkage when the assembly is fired and then allowed to cool. The use of a tube inherently limits the smallest possible feedthrough to the smallest available tubing. Acceptable results have been reported only when using tubes having a diameter >40 mils in ceramic substrates at least 70 mils thick. High temperature co-fired ceramics (HTCCs) are typically constructed with tungsten-based metallization fired in reducing atmosphere. When the tungsten is replaced with platinum, particularly in filled vias, the firing process is complicated by the undesired interaction of platinum with the ceramic-glass system. A platinum system has a greater thermodynamic tendency than does tungsten towards reduction of silica resulting in the volatilization of silicon monoxide and the formation of low melting temperature platinum silicides. These reactions respectively lead to devitrification of glass and abnormal microstructure evolution in liquid phase sintering ultimately producing the following functional defects: loss of via hermeticity, loss of electrical continuity, and loss of high temperature stability.
Additionally, since platinum is more resistant to oxidation than is tungsten, a platinum-based system may be fired in an atmosphere that would be considered oxidizing (i.e. air). However, in an oxidizing environment, the formation of volatile platinum oxides would be possible at high temperatures. This would lead to the following functional defects: loss of hermeticity and loss of high temperature stability.